This invention relates generally to the field of construction materials, particularly boards or panels, patching materials, joint compounds, and the like, which are made with gypsum and cement. Such materials may include more gypsum than cement, but still have good water resistance and strength.
Both gypsum and Portland cement (generally hereinafter "cement") are well known as construction materials. Gypsum (calcium sulfate dihydrate) is the principal component of the familiar wallboard, where it is faced with paper to provide strength and a smooth surface. Cement is used in various applications where its hardness, water resistance, and durability make it valuable, such as in concrete structures. Cement is also used in building panels where its hardness and water resistance are important.
Gypsum is generally produced by the rapid hydration of calcium sulfate hemihydrate, while Portland cement operates mainly by the relatively slower hydration of calcium silicate and aluminate minerals. Consequently, adding calcium sulfate hemihydrate to cement offers the benefits of improving the productivity of facilities which manufacture cement-containing panels, since the mixture hardens rapidly. Gypsum is, however, somewhat soluble in water, and mixtures which include both gypsum and cement are not as water resistant as cement alone or cement containing a minor amount of gypsum. Furthermore, it is well known that gypsum reacts with one of the components of cement, namely, tricalcium aluminate (3CaO.Al.sub.2 O.sub.3, abbreviated as C.sub.3 A) to form ettringite [3CaO.Al.sub.2 O.sub.3.(CaO.SO.sub.3).sub.3.32H.sub.2 O also C.sub.6 AS.sub.3 H.sub.32 ], which may cause expansion and undesirable cracking. Formation of ettringite can be useful, provided that it occurs early in the process of making panels (referred to as primary ettringite), since it provides fast setting and early mechanical strength. Once the mixture of gypsum and cement has been solidified, however, the formation of ettringite (referred to as secondary ettringite) is generally not desirable. Consequently, efforts have been made to prevent the formation of secondary ettringite in gypsum and cement formulations. This has been referred to as preventing an internal "sulfate attack," since it is the reaction of gypsum, CaSO.sub.4.2H.sub.2 O, with tricalcium aluminate and water, which results in the formation of ettringite. The tricalcium aluminate is quite soluble and cement often includes a small amount of gypsum to react with dissolved C.sub.3 A. A high alumina content does not necessarily mean that a cement is susceptible to sulfate attack, because the reactivity of the alumina-bearing compounds matters more than the total alumina content.
An important approach to limiting the formation of ettringite has been to add "pozzolanic" materials. In general, pozzolanic materials are defined by ASTM C618-97 as " . . . siliceous or siliceous and aluminous materials which in themselves possess little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties." For a given pozzolanic material, the finer the material is, the greater the pozzolanic activity. Also, amorphous materials are considered to possess greater pozzolanic activity. Finely divided amorphous silica, such as silica fume, has been found to have substantial pozzolanic activity. A related material, microsilica, is even more pozzolanic than silica fume. A crystalline silica having a large particle size, such as sand, would not be expected to have significant pozzolanic activity. Other naturally derived materials which, when finely divided, have been referred to as pozzolanic include pumice, perlite, diatomaceous earth, tuff, trass, etc. Man-made pozzolanic materials include metakaolin, microsilica, silica fume, ground granulated blast furnace slag, and fly ash.
Differences in pozzolanic activity may be related to the chemical reactivity of the components. That is, the quantity of silica or alumina in a pozzolanic material may not be as significant as the form in which they are found. The literature suggests that the temperatures used in processing of naturally derived or man-made pozzolans may determine whether or not the product is an active pozzolan. Thus, a high silica content may not be necessary, provided that the silica has been activated by its processing. Similarly, the alumina content of pozzolanic materials has been contended to be important. But, a high alumina content may have little effect, provided that the aluminum compounds are not reactive. For example, metakaolin contains less silica and much more alumina than silica fume, but has been found by the present inventor to provide superior mechanical performance in products made with metakaolin. Similarly, blast furnace slag contains less silica, but is more active than fly ash. It may be concluded that the pozzolanic activity of silica and alumina-containing materials should be considered only as potential until the pozzolanic properties are validated by appropriate tests.
In U.S. Pat. No. 4,494,990, Harris disclosed the effect of adding a pozzolanic material to a mixture of alpha gypsum (alpha hemihydrate) and Portland cement. He used a "sulfate reactivity factor" to determine whether the pozzolanic material was useful. This sulfate reactivity factor requires knowledge of the amounts of various components of the cement present and the relative amounts of the pozzolanic material, gypsum and cement. Broadly, the composition of Harris would contain 25-60 wt % of cement, 40-75 wt % of calcium sulfate hemihydrate (typically the alpha form), and 3-50 wt % of a pozzolan, in his examples, silica fume, having a sulfate reactivity factor less than 12.
In their article in Cement and Concrete Research, Vol. 25, No. 4, pp. 752-758, 1995, Singh and Garg discussed their work with a binder made of calcined phosphogypsum, fly ash, hydrated lime, and Portland cement.
In the same journal, Vol. 28, No. 3, pp. 423-437, 1998, Kovler reported on his work with blends of gypsum, Portland cement, and silica fume. Kovler stated "[s]uch blends can possess the advantages of gypsum (early hardening, high early strength, enhanced workability) and Portland cement (improved durability in moist conditions), but are free of the deleterious effect of ettringite and thaumasite, which are formed when gypsum and Portland cement react." (Note that "gypsum" is often used to refer to hemihydrates, as is done here.)
Bentur, Kovler and Goldman reported on similar compositions in Advances in Cement Research, Vol. 6, No. 23, pp. 109-116, 1994. They tested mixtures of gypsum, Portland cement, and silica fume having more than 92 wt % silica and noted that improved wet strength " . . . was explained by the reduction in ettringite formation and the development of a microstructure in which gypsum crystals were engulfed by CSH." By CSH was meant calcium silicate hydrate, according to a shorthand notation commonly used in the field, which constitutes the main constituent of Portland cement.
A high early strength cement was disclosed in U.S. Pat. No. 4,350,533 by Galer et al. of United States Gypsum Company. The high strength was obtained by forming substantial amounts of ettringite from mixtures containing high alumina cement and calcium sulfate (all forms, including gypsum, said to be useful). Pozzolanic materials, such as fly ash, montmorillonite clay, diatomaceous earth, and pumice, were considered optional ingredients, but could replace up to about 20% of the cement. A related and commonly assigned patent is U.S. Pat. No. 4,488,909.
In EP Patent No. 271,329, compositions containing 70% ettringite and up to 30% CSH were made using "non-traditional materials," including CaSO.sub.4.
In U.S. Pat. No. 4,661,159, Ortega et al. disclosed a floor underlayment composition which included alpha calcium sulfate hemihydrate (alpha gypsum), beta calcium sulfate hemihydrate (beta gypsum), fly ash, and Portland cement.
In U.S. Pat. No. 5,439,518, Francis et al. disclosed a composition including a fly ash containing up to 30% CaO which reacts with gypsum to form a cement. Optionally, up to 20% Portland cement may be added.
In U.S. Pat. Nos. 5,685,903 and 5,718,759, Stav et al. disclosed a composition which contains 20-75 wt % calcium sulfate beta-hemihydrate, 10-50 wt % of a cement selected from the group consisting of Portland cement, a blend of Portland cement and fly ash, and a blend of Portland cement and ground blast slag, and mixtures thereof, 4-20 wt % silica fume, and 1-50 wt % of a pozzolanic aggregate. The aggregate was defined as having an average particle size larger than that of Portland cement, i.e., larger than 45 microns. This contrasts with the silica fume, which is an active pozzolanic material and has a much smaller particle size, said to be about 0.1-0.3 microns. The Stav et al. composition was contended to have improved water resistance and higher compressive strength compared to similar compositions which used aggregates considered to be non-pozzolanic, such as sand, clays and calcium carbonate.
In U.S. Pat. No. 5,858,083, Stav et al. stated that the composition of silica fume produced from silicon production is important for achieving the desired results. The maximum alumina content was said to be 0.6 wt %, and the minimum amorphous silica to be 92 wt %.
The effect that aggregates may have on the strength of the composition, which Stav et al. attributed to pozzolanic activity, may instead relate to other factors, such as the shape, size, and gradation of the particles, mechanical packing, roughness, water absorption, etc.
Silica fume and other pozzolans have been suggested as replacements for a portion of cement to impart various properties. Addition of materials that potentially may be classified as pozzolanic does not, however, necessarily result in greater strength. In a publication entitled "Pozzolanic Reactivity Of Lightweight Aggregates," Cement and Concrete Research, Pergamon Press plc, Vol. 20, pp. 884-890, 1990, Zhang and Gj.o slashed.rv studied the reactivity of expanded clay and fly ash aggregate having a silica content of about 50-60 wt % and an alumina content of 17-27 wt %. The expanded clay and fly ash aggregates were ground to a Blaine fineness of about 4,000 cm.sup.2 /g, thus, their reactivity would be expected to have been much enhanced as compared with their use as aggregates. Nevertheless, the ground clay and fly ash aggregate were practically non-reactive and the authors concluded "a significant effect of pozzolanic reaction between cement paste and lightweight aggregates should not be expected." Zhang and Gj.o slashed.rv studied the effect of the temperature at which clay aggregates were fired on compressive strength of concrete. They concluded that "the low degree of pozzolanic reactivity may be the result of a recrystallization of the mineral compounds during the manufacturing process of the aggregate." In contrast, silica fume showed a significant degree of reactivity. The silica fume was much finer (19.8 m.sup.2 /g) and had 91.7 wt % SiO.sub.2 and 0.2 wt % Al.sub.2 O.sub.3.
A. Goldman and A. Bentur found that silica fume has a strengthening effect on concrete which related to its ability to act as a microfiller, rather than to its chemical reactivity. "Properties Of Cementitious Systems Containing Silica Fume Or Nonreactive Microfillers," Advn Cem Bas Mat, Elsevier Science, Inc., [1], pp. 209-215, 1994.
The experience of others, as discussed above, did not provide an answer to the problem faced by the present inventor. Namely, how to substitute gypsum for cement in panels used for applications where water could be expected, such as panels supporting ceramic tiles, underlay, and the like. Currently, panels having a large cement content are used, but these are heavy and costly, and lighter panels which still retain the necessary water resistance are desired. Another essential characteristic of such panels, however, is that they must not expand to exceed the strain capacity of other elements, such as tiles, joints, fasteners, studs, etc., when in the presence of water. The tiles, panels or joints will crack, leading to further water intrusion and deterioration of the materials. Generally, the expansion in water of materials made with gypsum--cement is too large to be acceptable. The present inventor has found a solution to the problem, thus permitting manufacture of lighter weight, water resistant panels which, when in contact with water, expand only slightly and do not exceed the strain capacity of the other elements.